No prior art exists today for rendering spent nuclear fuel harmless in large scale. Hence, various concepts have been developed in order to bury the spent nuclear fuel in primary rock. The spent nuclear fuel may have to rest in the primary rock for hundreds of thousands of years before the radioactivity has abated to a level that is harmless to man and animal. It is important during this long period of time to ensure that the fuel does not dissolve such that radioactive particles rise to ground surface via the ground water.
Water is needed in order for the spent fuel to dissolve and spread. It is hence important for the final repository to ensure that the spent fuel is maintained encapsulated in a tight canister and thereby is prevented from contacting the surrounding water until the radioactivity has abated to a low level. In most of the hitherto developed concepts the final repository comprises a system of barriers (canister, buffer and rock) that together are intended to prevent the radioactive species of the fuel from reaching ground surface. If one barrier does not work as planned, the other barriers will nevertheless guarantee safety, according to SKB, the Swedish Nuclear Fuel and Waste Management Co (www.skb.se). The canister (with an insert) is closest to the fuel. It is this barrier that is primarily intended to isolate the fuel from the surroundings. The objective of the canister in the repository is to completely encapsulate the spent fuel for a very long time, since no radioactive species can reach ground surface as long as the canister is tight.
The Swedish Nuclear Fuel and Waste Management Co has developed a concept, the KBS-3 method, that is based on encapsulation of spent nuclear fuel in a protective copper casing that is thereafter embedded in bentonite clay at a depth of 500 meters in the primary rock. Hereby, the bentonite clay acts as a buffer against mechanical stress in the canister caused by rock movements and will also limit the ground water flow. Inside the protective copper casing there is a nodular iron insert in order to increase strength.
The copper casing is joined together by e.g. friction welding or electron beam welding or some other welding method. Countries such as Sweden, Finland and Canada are planning to deposit their spent nuclear fuel according to this concept or a similar one.
Copper is classified as a corrosion allowing metal in water that contains O2. SKB is of the opinion that the rate of corrosion in anoxic (free from gaseous oxygen) ground water only depends on accessible sulphur and that it hence is extremely low. It is thereby considered that the copper will give a perfect protection against corrosion until the radioactivity has abated. On page 28, second paragraph of the SKB report Encapsulation—When, where, how and why? (SKB Art. 141 2008) the following statement can be read: Today we really know all we need to know about corrosion in order to design the canister and the respitory to be safe for much more than 100,000 years. The present consensus about copper corrosion is also summarised in SKB Technical report Tr-01-23.
Other countries have chosen another concept according to which a corrosion resistant metal such as titanium, titanium alloys, high-alloy stainless steel or nickel alloys has been chosen for the casing, instead of a corrosion allowing metal. It is characteristic for such alloys that their excellent resistance to corrosion is achieved by the formation of a thin passive layer of oxide (so called passive film) that forms on the outer surface of the metal.
Irrespective of the choice of material for the canister casing, the demands are very high since they will be exposed to hard environments for a long time, such as:                ground water that contains sulphide and chloride.        ground water that contains O2 (oxidising) for 1-3,000 years.        Anoxic environment for the following 100,000-1,000,000 years.        Elevated temperatures (30-100° C.) for 10,000 years due to radioactive decay of the fuel.        
For the alloys that form passive films there is a certain risk of pitting corrosion, particularly if the chloride content of the ground water gets very high. This is a problem for the concepts based on the corrosion resistance of a passive layer. Accordingly, there is a risk that the barrier that is to be constituted by the canister is prematurely penetrated.
The applicant has surprisingly found that copper is not immune in water free from O2 (anoxic water). This means that the rate of corrosion of copper in contact with anoxic ground water is much higher than previously assumed. Moreover, it has surprisingly been found that the rate of corrosion of copper in an anoxic water environment is very temperature sensitive and completely unacceptable rates of corrosion are achieved at 60-90° C. with the formation of a high hydrogen-containing, porous and non-protective oxide. Such temperatures may exist for up to 10,000 years due to the activity of the fuel.
This means that none of the above mentioned concepts will result in a corrosion protection that, with adequate safety margins, will cope with the conditions that the canisters may be exposed to, until the radioactivity has abated to levels that are harmless to man and animal.
It should also be noted that the canisters in question for spent nuclear fuel are relatively large. The canisters according to the KBS-3 method are almost five meters high and have a diameter of slightly more than one meter. The casing consists of copper with a thickness of five centimeters. In order to enhance strength, it has an insert of nodular iron which is a type of cast iron, on the inside. When the canister is full of spent fuel it will weigh between 25 and 27 metric ton.
SE 425,707 and U.S. Pat. No. 4,834,917 are both based on hot isostatic pressing (HIP) of copper powder for the manufacturing of the outer canister. However, hot isostatic pressing of copper powder for this purpose has several drawbacks:                1) A HIP:ed copper powder may result in worse corrosion and mechanical properties than the KBS-3 method with hot formed (forged) OFP copper alloy as suggested by the SKB.        2) It is expensive and technically difficult to HIP such large canisters as are required by the KBS-3 concept (diameter of about 1 m, height of about 5 m).        3) All methods that require for the radioactive material to be positioned inside the inner canister during the sintering process/HIP are completely out of the question in order to achieve an improvement over the KBS-3 concept, since the fuel rods do not withstand high temperatures. That is because it is very hard to avoid oxidation of the main component of the spent nuclear fuel pellets, UO2, to higher oxides at elevated temperatures. Such a conversion is risky since it may result in a 35% volume expansion and pulverization of the pellets. In addition, the solubility of the higher oxides is too high in this context.        4) It is estimated that a certain percentage of the fuel rods (zircaloy tubes with uranium dioxide pellets) will contain water absorbed/adsorbed in the porous UO2 pellets during the interim storage that takes place in a water reservoir for 30-40 years (alternatively from the time period in the reactor). It is not considered to be technically/economically possible to dry the fuel rods after the interim storage, to a guaranteed dryness. This means that if the UO2 pellets are exposed to heat treatment they will rapidly react with moisture and water during heating, which must not take place (see paragraph 3).        
SE 509,177 shows an example on how to produce a canister for the KBS-3 method, i.e. a canister comprising an inner steel canister and an outer copper canister. The outer copper canister is formed by electrolytic copper coating of the inner steel canister.
U.S. Pat. No. 4,562,001 shows a container that comprises at least three layers of different metals, which, from the outside inwardly, are always more noble. In one example, the outer layer consists of cast iron, the intermediate layer consists of nickel or a nickel alloy, and the inner layer consists of copper or a copper alloy. The problem of having an outermost non-noble metal such as cast iron or carbon steel, is the strong development of gaseous hydrogen due to anoxic corrosion. In this corrosion reaction, the thermodynamic equilibrium pressure of hydrogen is about 700 bar (Thermo Calc software, SSUB-database 2006).
In all repository methods in which bentonite clay is to be used as external buffer around the metal canister (such as in KBS-3), there must not be any build-up of hydrogen gas pressure since that may ruin the buffer protection by the formation of holes and channels in the bentonite clay, which in turn results in far too fast transport paths for gas, water and ions in the same. The hydrogen released from anoxic corrosion of iron furthermore results not only in hydrogen gas (H2) but also in a considerable amount of atomic hydrogen (H) that migrates into the metal. This electrochemical hydrogen load due to corrosion of iron is harmful also to nickel and copper alloys that are not per se classified as particularly hydrogen sensitive alloys. The problem is that hydrogen load due to corrosion of iron can be said to correspond to a hydrogen gas pressure in the magnitude of 700 bar. A strong hydrogen activity will degrade virtually all metals (including copper and nickel alloys) in terms of mechanical properties (strength, creep ductility, toughness, etc.), and in addition the corrosion resistance will decrease for copper and nickel alloys at the same time as the risk of stress corrosion cracking (SCC) and hydrogen embrittlement increases.
Contrary to U.S. Pat. No. 4,562,001, the applicant's patent is based on a copper canister with an outer metal alloy that forms a passive film based on chromium oxide, zirconium oxide or titanium oxide. Note that copper is less noble in the electromotive series than the oxide forming alloys containing Zr, Ti, Cr in their passive state (normal state).